This invention pertains to an apparatus and a method of bonding an active tag to a patch and to a tire. More particularly, this invention sets forth a method and apparatus for encapsulating an electronic device in a rigid material, mounting the device on a vulcanized rubber patch and bonding the rubber patch to a vulcanized tire.
It is desirable to monitor engineering conditions of tires, such as wear, internal pressure and internal temperature in order to reduce tire costs and maximize vehicle efficiency. Of course, it is advantageous to perform such monitoring in large truck tires, which are expensive.
Prior art methods of monitoring large truck tires have included passive integrated circuits embedded in the body of the tire, or self-powered circuits which are positioned external to the tire. The passive integrated circuits rely on inductive magnetic coupling or capacitative coupling to energize the circuit, thus providing power to the circuit from a source remote from the tire. Self-powered circuits positioned external to the tire are exposed to damage from the environment such as weather, road hazards and even vandalism.
Recent engineering advances have permitted the installation of monitoring devices having active integrated circuits within tires. One such device is described in U.S. Pat. No. 5,562,787 to Koch et al. entitled xe2x80x9cMethod of Monitoring Conditions of Vehicle Tiresxe2x80x9d, incorporated herein by reference, and assigned to the assignee of the present invention. These devices include an active circuit powered by a dedicated long life, miniature battery and at least one sensor for detecting, optionally storing and transmitting real time engineering conditions within the tire. Such devices are capable of being programmed to remain in an active, but dormant condition, but will switch automatically to an xe2x80x9cawakenedxe2x80x9d condition in response to an external signal or a condition which exceeds preset limits.
One of the problems faced with such active devices is that they are delicate electronic devices that must operate in the harsh environment of a tire. Thus it is important to secure these devices in the tires to minimize the effect of the harsh tire environment on them, while still permitting them to be exposed to this environment to allow accurate monitoring of the engineering conditions for the life of the tire. These active devices have previously been mounted in tires by first encapsulating the device or power-containing circuit in a material which forms a rigid or semi-rigid encasement about the device, thereby inhibiting straining of the device as a result of applied stresses. Such materials have included non-foam compounds such as urethanes, epoxies, polyester-styrene resins, hard rubber compositions, and the like. The encapsulated device is then placed into a green rubber material which forms a housing or is placed in a green rubber pocket or pouch that becomes part of the tire. The encapsulated device is then permanently set within the rubber material during a subsequent vulcanizing operation. The green rubber material containing the encapsulated device may be assembled to the green tire and then vulcanized with the tire. Alternatively, the green patch containing the encapsulated device may be separately vulcanized and then affixed to a cured tire with a suitable adhesive. In either event, the delicate electronic components of the active tag are exposed to high vulcanizing temperatures which may adversely affect their performance or shorten their life. Additionally, certain components of the device, such as the pressure sensor or a thermistor must remain open to the pressurized tire cavity. The flow of rubber occurring during the vulcanization process can effect the openings to the tire cavity. As set forth in U.S. Pat. No. 5,562,787, the opening to the tire cavity can remain open during the curing operations by placement of a dowel into the opening.
While the methods and apparatus of U.S. Pat. No. 5,562,787 provide an acceptable method of assembling an active chip into a tire cavity, an improved method and apparatus which increases the life of the active chip in the tire is desirable.
In accordance with the present invention, an improved method and apparatus for assembling an active electronic monitoring device into a tire cavity is disclosed.
The improved method and apparatus increases the life of the electronic monitoring device by minimizing the stress, strain, cyclic fatigue, impact and vibration to which the electronic monitoring device is subjected when properly assembled into a tire. While the location of the device is an important factor in determining the life of an electronic monitoring device located within a tire, so too is the manner in which the device is installed into the tire. A method for securing within a tire an electronic monitoring device for monitoring at least one engineering condition of a tire comprises the steps of encapsulating an electronic monitoring device by placing the electronic monitoring device into a mold having a preselected configuration, the preselected configuration including at least one face having an alternating raised and recessed pattern to increase the surface area of the face. The mold is filled with an epoxy potting material so that the electronic monitoring device is encased by the epoxy. The epoxy potting material is cured to form a rigid tag assembly having a preselected configuration. However, certain electronic components comprising the electronic monitoring device, such as pressure sensors, are assembled in such a manner as to remain free from internal contamination while remaining open to the tire atmosphere as necessary. A rubber tire patch having a preselected configuration is formed. The rubber patch includes a first side for interfacing with the face of the encapsulated rigid tag assembly. This first side has increased surface area, which allows for a better bond with the encapsulated tag. The first side also includes a recessed cavity surrounded by a ridge of material of sufficient height to capture the encapsulated rigid tag assembly This ridge helps to improve the bond between the rubber patch and the encapsulated rigid tag. The patch includes a second opposite side, the second side approximating the contour of the inner liner of the tire. The patch is tapered from the first side toward the second side, with the second side flaring out into a relatively thin configuration, but having a radius approaching the radius of the tire inner liner. The rubber patch is vulcanized at a preselected temperature and for a time sufficient its vulcanization. After vulcanization, a thin layer of dual cure bonding rubber is applied to the second side of the vulcanized rubber patch. This thin layer of dual cure bonding rubber will permit the assembly of the patch assembly to the inner liner of the tire. A fluid epoxy adhesive is applied to an interface between the encapsulated tag assembly and the rubber patch. The encapsulated tag assembly is assembled into the recessed cavity on the first side of the rubber patch so that the fluid epoxy adhesive is evenly distributed across the interface and flows from the interface between the encapsulated tag assembly and the rubber patch. Of course, the increased surface area of the tag assembly allows for better bonding between the rubber patch and the encapsulated tag. The assembly or the encapsulated tag and the rubber patch is allowed to cure to form a patch assembly. The patch assembly is then assembled to the inner liner of a vulcanized tire by applying an activating cement between the dual cure bonding layer and the inner liner of the tire. This curing process permanently bonds the patch assembly to the inner liner and is diffusion-controlled. Because it is diffusion controlled, the curing must be done for a sufficient amount of time and at a sufficient temperature to allow a strong permanent bond to form. To assure that solid contact is maintained at the interface between the inner liner of the tire and the dual cure bonding layer of the patch assembly, a clamping mechanism is applied to secure the patch assembly to the inner liner during the curing process. The clamping mechanism is applied with sufficient pressure across the interface of the patch assembly and the inner liner to assure intimate contact during curing.